The Mesoamerican ruins of Teotihuacan, a World Heritage Site, represent one of the largest urban centers of the ancient world. Thought to have been established around 100 B.C., the pyramid-filled city had more than 100,000 inhabitants at its peak, but was abandoned for mysterious reasons around 700 A.D. — long before the Aztecs arrived in the 1300s.

The excavation at the temple focused on a 330-foot-long tunnel which runs under the structure. The conduit was discovered in 2003 when heavy rain uncovered a hole a few feet from the pyramid.

Exploring the tunnel, which was deliberately filled with debris and ruins by the Teotihuacan people, required several years of preliminary work and planning.

"Finally, a few months ago we found two side chambers at 72 and 74 meters (236 and 242 feet) from the entrance. We called them North Chamber and South Chamber,” archaeologist Sergio Gómez Chávez, director of the Tlalocan Project, told Discovery News.

The archaeologists explored the tunnel with a remote-controlled robot called Tlaloc II-TC, which has an infrared camera and a laser scanner that generates 3-D visualization of the spaces beneath the temple.

"The robot was able to enter in the part of the tunnel which has not yet been excavated yet and found three chambers between 100 and 110 meters (328 and 360 feet) from the entrance," Gómez Chávez said.

The mysterious spheres lay in both the north and south chambers. Ranging from 1.5 to 5 inches, the objects have a core of clay and are covered with a yellow material called jarosite.

DMC, INAH/M. Marat

The Temple of the Feathered Serpent, also known as the Temple of Quetzalcoatl, is a six-level pyramid decorated with snake-like creatures.

"This material is formed by the oxidation of pyrite, which is a metallic ore," Gómez Chávez said. "It means that in pre-hispanic times they appeared as if they were metallic spheres. There are hundreds of these in the south chamber."

According to George Cowgill, professor emeritus at Arizona State University and the author of several publications on Teotihuacan, the spheres are a fascinating find.

"Pyrite was certainly used by the Teotihuacanos and other ancient Mesoamerican societies," Cowgill told Discovery News. "Originally the spheres would have shown brilliantly. They are indeed unique, but I have no idea what they mean."

Even the walls and ceiling of both chambers were covered with a mineral powder composed of magnetite, pyrite and hematite which provided a special brightness to the place.

"We believe that high-ranking people, priests or even rulers, went down to the tunnel to perform rituals," Gómez Chávez said.

Indeed the archaeologists found many offerings, including pottery and wooden masks covered with inlaid rock crystal, jade and quartz — all dating from around 100 A.D.

Gómez Chávez and his team now look forward to the next phase of the project — exploring the last part of the tunnel and three chambers which archaeologists have seen through the robot cameras.

"The tunnel is in pristine condition, untouched for almost two millennia," said Ng “TC” Tze Chuen, an independent researcher who worked on the design of the Tláloc II-TC robot. "Can you can imagine what can be found inside?"

Ng, who helped create the Djedi robot that explored Egypt's Great Pyramid in 2010, believes the Mexican tunnel might lead to one of the most significant archaeological finds in Teotihuacan.

"The results are very encouraging indeed," he said.

According to Gómez Chávez, the tunnel was sealed twice by the Teotihuacan people. Thick walls, erected to block access, were demolished about 1,800 years ago in order to deposit something very important in the central chamber at the end of the tunnel.

"Maybe in this place," Gómez Chávez said, "we will find the remains of those who ruled Teotihuacan.”